1. Field of the Invention
The present invention relates to packet switching in communication networks. More specifically, the present invention relates to a method and apparatus for switching packets in a passive optical network.
2. Related Art
In order to keep pace with increasing Internet traffic, optical fibers and optical transmission equipment have been widely deployed to substantially increase the capacity of backbone networks. However, this capacity increase in backbone networks has not been matched by a corresponding capacity increase in access networks. Even with broadband access solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks creates a severe bottleneck in delivering high bandwidth to end users.
Among the different technologies presently being developed, Ethernet passive optical networks (EPONs) are among the best candidates for next-generation access networks. EPONs combine ubiquitous Ethernet technology with inexpensive passive optics. They offer the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. Because of optical fiber's high bandwidth, EPONs can support broadband voice, data, and video traffic simultaneously. Such integrated services are difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, since Ethernet frames can encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. This first mile network is a logical point-to-multipoint network, with a central office servicing a number of subscribers. In a typical tree-topology EPON, one fiber couples the central office to a passive optical coupler, which divides and distributes downstream optical signals to subscribers. The coupler also combines upstream signals from subscribers (see FIG. 1).
Transmissions in an EPON are typically between an optical line terminal (OLT) and optical networks units (ONUs) (see FIG. 2). The OLT generally resides in the central office and couples the optical access network to an external network (e.g., a carrier network). An ONU can be located either at the curb or at an end-user location, and can provide broadband voice, data, and video services. ONUs are typically coupled to a one-by-N (1×N) passive optical coupler, and the passive optical coupler is coupled to the OLT through a single optical link. (Note that a number of optical couplers can be cascaded.) This configuration can achieve significant savings in the number of fibers and amount of hardware.
Communications within an EPON are divided into downstream traffic (from OLT to ONUs) and upstream traffic (from ONUs to OLT). In the upstream direction, the ONUs share channel capacity and resources, since there is only one link coupling the passive optical coupler with the OLT. In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, packets are broadcast by the OLT to all ONUs and are subsequently extracted by their destination ONUs. Each network device attached to a certain ONU is assigned a Logical Link ID (LLID), according to the IEEE 802.3ah standard. A downstream packet is first processed at the OLT, where the packet receives the LLID of its destination, and is then transmitted to the ONUs. Although a packet is broadcast to all the ONUs, only the ONUs with an LLID that matches the one with the packet is allowed to receive the packet. Therefore, the OLT switches packets by attaching proper LLIDs to the packets.
The OLT is responsible for processing and forwarding both upstream and downstream packets. Because the links in EPONs may operate at high speeds (e.g., 1 Gbps and above), the OLT bears the burden of processing a large number of packets at line speed. Furthermore, customers often request virtual local area network (VLAN) services. The additional VLAN packet processing adds to OLT's workload.
Hence, what is needed is a method and apparatus for switching packets in a passive optical network, which can process packets at line speed and implement various forwarding policies cost-effectively.